Advanced processing of audio signals has become increasingly important in many areas including e.g. telecommunication, content distribution etc. For example, in some applications, such as hands-free communication and voice control systems, complex processing of inputs from a plurality of microphones has been used to provide a configurable directional sensitivity for a microphone array comprising the microphones. Specifically, the processing of signals from a microphone array can generate an audio beam with a direction that can be changed simply by changing the characteristics of the combination of the individual microphone signals.
Typically, beam form algorithms seek to attenuate interferers while providing a high gain for a desired sound source. For example, a beamforming algorithm can be controlled to provide a strong attenuation (preferably a null) in the direction of a signal received from a main interferer.
For practical reasons it is desirable that the microphone array is relatively small. However, when the wavelength of the sound of interest is much larger than the size of the array, many beamforming algorithms, such as additive delay-and-sum beamforming algorithms, are not able to provide sufficient directivity as the beamwidth deteriorates substantially for such wavelengths.
One approach for achieving an improved directivity is to apply so called superdirective beamforming techniques. Such superdirective beamforming techniques are based on filters with asymmetrical filter coefficients and the approach essentially corresponds to subtraction of signals or determining spatial derivatives of the sound pressure field. However, although this may improve the directivity, it is also known that this is achieved at the expense of robustness, such as increased sensitivity to white (sensor) noise and an increased sensitivity to mismatches in microphones characteristics.
In the article “Optimal Azimuthal Steering of a First-order Superdirectional Microphone Response” by R. M. M. Derkx, International Workshop on Acoustic Echo and Noise Control, September 2008, Seattle, a system is analyzed which generates Eigenbeams for a two dimensional microphone array. The Eigenbeams are then combined to maximize the attenuation of a single point interference source. In particular, a null is located in the direction of a single point interferer while maintaining a suitable gain for the desired direction.
However, although this approach provides improved performance in many scenarios, it provides non optimal performance in some practical scenarios. It also tends to require relatively complex and resource demanding processing.
Hence, an improved approach for audio beamforming would be advantageous and in particular an approach allowing improved adaptation to current conditions and audio environment, increased flexibility, facilitated implementation, improved performance for different operating scenarios and/or improved performance would be advantageous.